1. Field of the Invention
This invention relates to the field of video signal processing and, more particularly, to the processing of color video signals to compensate for the effect of sensor overload.
2. Description Relative to the Prior Art
Image sensors, particularly charge-coupled device (CCD) sensors, are driven into a non-linear region of response when the photosites are brightly illuminated and, in the case of CCD sensors, the charge levels approach the maximum well capacity. This means that the output signal from the sensor does not respond proportionately to the input light. In such a condition, the sensor is said to be overloaded, at least for the sensor photosites that experience such a response. Color image sensors, of course, are sensitive to several constituent colors which are combined in subsequent video processing to form an output video signal representative of the color of the input light. Furthermore, a given color is ordinarily made up of unequal amounts of the constituent colors. Since, therefore, for high illumination the charge residing in the different color-sensitive photosites will be driven non-linear at different points, the resultant color represented by the output video signals shifts away from the desired color. Moreover, the color shift is unpredictable. This produces visually unappealing colors in local areas of the reproduced image.
It is known how to monitor the level of the color signals from the sensor and to reduce a composite output video signal in some controlled fashion when overload is detected. For example, Sony Corporation has disclosed an NTSC processor circuit that provides dynamic color compensation for an NTSC signal. The several color signals from the sensor are compared to a threshold. When the threshold is exceed by any one of the color signals, the modulated chrominance signal is accordingly reduced or set to some predetermined level. The result is to produce a white output for the affected image points. It has been found that an ordinary viewer tolerates a white error, rather than some arbitrary color, and image quality is thereby preserved. It is further possible to fade the whole NTSC signal under certain situations, such as when a recording operation stops.
Color compensation systems operating in modulated chrominance space cannot be conveniently adapted to other color systems. In particular, it would be desirable to provide overload compensation for systems producing baseband signals, such as systems producing red, green, blue (RGB) or luminance and color difference signals (Y, R-Y, B-Y).